Bpftrace: Improve logging of bpftrace

[suyuee]

Got these ideas from @fbs and @danobi.

Is your feature request related to a problem? Please describe.

Generally, it would be nice to provide logging facilities to developers and users so that they could have better visibility of bpftrace internal states.

1. bpftrace currently doesn't provide tools for developers to print debug messages. It would be helpful if we can have some handy tools.
2. There should be semantic warnings for users when they try to do things unproperly and having a consistent way to print the warnings can make it easier to do so. Also, bpftrace should allow users to enable and disable warnings like gcc does.
3. bpftrace should provide options about what kind of debug information to print, as in #131.

Describe the solution you'd like

Implementing a logging framework for bpftrace. It would be much easier to deal with these issues with the right infrastructure.

• [x] Add the logging infra. #1418
• [x] Replace callsites of class BPFtrace logging facilities (warning() & error()). #1443
• [x] Add the option to turn off warnings (maybe with -w as in gcc).#1444
• [ ] Add the option to show certain debugging information (ast, ir).
• [x] Change the error/warning messages sent through std::cerr.#1449

Describe alternative solutions or features you've considered

N/A

[danobi]

In other projects I've really liked macro style loggers. For example:

LOG(ast) << "i'm an ast log";
LOG(codegen) << "i'm a codegen log";
LOG << "default log";